1. Field of the Invention
The present invention relates to testing systems for coupled joints. More specifically, the invention relates to a biomechanical system that can controllably test portions of a human spine or other coupled joints along multiple axes and degrees of freedom.
2. Description of the Related Art
The spinal implant device industry has experienced significant growth in recent years. Examples of such growth include the development of disc replacement devices which may be implanted into a patient's spine. Tissue implants are also being developed. The goal of such disc arthroplasty and other implants is to restore normal joint height, stability and physiological movement within the patient.
As with other medical devices, disc arthroplasty and other implant devices must undergo in vitro testing before they can be placed within a patient. Such testing is often required before clinical trials by the Food and Drug Administration (FDA). Ideally, testing would be in a manner that is reflective of the combined motion and loading patterns within the human musculoskeletal structure. However, state-of-the-art testing devices are limited in their ability to reproduce a full range of human movements. Further, state-of-the-art testing devices may not accurately apply loads in a multi-axial environment. In this respect, the relative movement between adjacent spinal vertebrae defines a complex, highly mobile articulating system that is difficult to reproduce.
The spinal articulating system provides for three-dimensional motion between each vertebra. FIG. 1 illustrates individual vertebra movements with reference to the anatomical planes. Six degrees of freedom are demonstrated representing vertebral motion.
In FIG. 1, a single motion segment unit of two vertebrae 10 and interconnecting disc 15 is shown. The vertebra 10 has an anterior side 14 and a posterior side 12. Three anatomical axes defined as “x,” “y,” and “z” are provided. The three axes “x,” “y,” and “z” converge at the vertebra 10. The “y” axis represents an axis about which flexion and extension occur; the “z” axis represents a vertical axis about which axial rotation occurs; and the “x” axis represents an axis about which lateral (or side) bending occurs.
The “x,” “y,” and “z” axes also form three anatomical planes (not indicated). The plane defined by the “z-y” axes is the frontal plane; the plane defined by the “x-z” axes is the sagittal plane; and the plane defined by the “x-y” axes is the transverse plane. Flexion and extension occur within the sagittal plane, while lateral bending occurs within the frontal plane.
It is also understood that the vertebra 10 is capable of translation along the “x,” “y,” and “z” axes. Thus, the six degrees of freedom, or DOF, represented in FIG. 1 are (1) translation (or displacement) along the “x” axis; (2) rotation about the “x” axis; (3) translation along the “y” axis; (4) rotation about the “y” axis; (5) translation along the “z” axis; and (6) rotation about the “z” axis.
In actual physiological conditions, the vertebra 10 will undergo forces and moments about the “x,” “y,” and “z” axes. FIG. 2 presents a human head 20 exhibiting lateral bending motion. This means that the head 20 is rotating within the frontal plane of FIG. 1. To accommodate bending, a cervical spine 25 is shown. In the left view 22, the head 20 and spine 25 are being bent to the left; in the center view 24, the head 20 and spine 25 are in a neutral position; and in the right view 26, the head 20 and spine 25 are being bent to the right. It is noted in the left-bending 22 and right-bending 24 views, the head 20 and spine 25 are also being rotated axially about axes that lie within the frontal plane and move with the spine as it laterally bends. This is because maximal axial rotations cannot occur without lateral bending, and visa-versa. Arrows indicate these combined movements. Similarly, the range of flexion-extension motion permissible is dependent on the degree of left-right axial rotation.
It is desirable to have laboratory testing systems that are capable of replicating the multi-axial motions (such as motions 22 and 26 shown in FIG. 2) and loads experienced in vivo. However, because of the kinematic complexities involved in defining and testing spinal movement, researchers have often simplified analysis of spinal movements to a two-dimensional case. In a two-dimensional (2-D) study, motion is constrained to only one of the anatomic planes, typically the sagittal plane. The sagittal plane exhibits the least out-of-plane motion and is the most common choice for 2-D analysis.
Another shortcoming to most testing machines in commercial use today is that they apply a pure bending moment to a spinal test specimen. The relative vertebral rotations induced at the various spinal levels due to applied pure moments do not agree with those observed in vivo. For example, systems that load the sub-axial cervical spine using a pure moment load induce the greatest flexion-extension motion towards the outer spinal bodies, that is, C3-C4 and C6-C7. This is in direct contrast to a multitude of in vivo data in which the greatest motions have been reported to occur in the mid-spine region, typically at C4-C5 and C5-C6. Thus, commonly employed pure moment protocols do not replicate sagittal plane in vivo spinal kinematics. In addition, and as noted above, such a two-dimensional testing system only replicates a small portion of actual human movements.
Previous efforts towards robotics-based testing of the spine have been limited to either pure displacement control methods, or slow, quasi-static force control approaches. Pure displacement control methods involve application of predetermined motions to a test specimen and the measurement of resulting forces. Quasi-static force control approaches involve an iterative method whereby a small incremental displacement is applied to a spinal body and the forces measured. The position of the spinal body being moved is then readjusted by small amounts in specific directions according to a governing algorithm until predetermined spinal body loading criteria are met or optimized. A subsequent small incremental displacement is applied and the process is repeated. The adjusted location points which meet the force loading criteria thus form a motion path or range of motion that can be stitched together and replayed by the robot in one large continuous motion over which spinal loading is intended to remain within some specified parameters and tolerance. Load control methods that allow a spinal test specimen to move inherently ‘where it wants to’ as opposed to some prescribed path are preferred. Because most scientific investigations of spinal joints and/or added instrumentation involve comparison of different spinal conditions subjected to an identical loading input, pure displacement control methods have limited investigational use. Quasi static load control approaches are cumbersome and extremely time consuming to complete one full spinal body motion over a full physiologic range. Further, because of the stop start nature of quasi-static tests, the full dynamic nature and response of the tissue under investigation may not be realized.
None of the conventional laboratory testing systems is capable of replicating or evaluating the combined motion and loading patterns associated with the human spine. In vivo physiologic spinal movements cannot be fully tested or described by motions along a single axis or within a single plane. In this respect, in vivo physiologic spinal movements involve motion in more than one plane. Further, conventional laboratory test systems are typically programmed around independent control of each available degree of freedom. Conventional laboratory systems as such do not possess a sufficient number of, and appropriate composition of, controllable degrees of freedom that can be simultaneously coordinated to input motion and force parameters that can induce a full range of physiologic and coupled spinal movements.
Therefore, a need exists for an in vitro spinal testing system that enables the simulation of in vivo vertebral body motion in space, and that can maintain target end loads in real time. Further, a need exists for a multi-axis, programmable, in vitro testing system for tissue and implant testing that is capable of investigating coupled joint movements. Still further, a need exists for a programmable, in vitro testing system that applies specified end loads to a spinal specimen throughout a physiologic range of motion.